The present invention relates to an optical data link and more particularly to a circuit which incorporates a single light-emitting diode for both transmitting and receiving information.
Fiber optic technology has resulted in the extensive use of fiber optic light pipes or bundles for transmitting data. The information to be transmitted is converted from electrical energy to light energy which is then transmitted through a fiber optic bundle to a receiver where the light energy is converted into electrical energy. Fiber optic bundles are chosen for data links rather than shielded cable for numerous reasons including noise immunity from electromagnetic interference and high transmission speed. Compared with shielded cable, fiber optic bundles have increased strength and flexibility, higher reliability, and fewer connector problems associated with low level signals.
For light communications between two or more locations, a transmitting device, such as a light-emitting diode (LED), and a separate light receiving device are used for converting between light and electrical energy. Often a different fiber optic bundle is used for transmitting in one direction than is used for transmission in the opposite direction. Two optic couplers are required at each location, one for coupling the LED light output to a bundle and one for coupling the light from the bundle to the receiver processing circuitry. If a single fiber optic bundle or pipe is used, each end of the bundle or pipe is usually split to facilitate coupling to the separate transmitting and receiving devices in the circuitry at each location. Using the separate transmitting and receiving devices increases the number of optical couplers and, often, the number of fiber optic bundles required for a data link, resulting in the increase of bulk and cost of the system.
Attempts to provide a conditioning circuit with a single solid state device to both transmit to and receive information from a light transmitting media, such as a fiber optic bundle, have heretofore generally been unsuccessful. Semiconductor devices which provide an adequate light output for transmission usually have a very low current output upon receipt of a light signal. Problems of stability have been encountered because the input upon receipt of light is generally very low. The input offset current of the amplifier used is an important limiting factor. The low input signal also requires a very high gain amplifier with good stability characteristics. In addition, the input amplifier must have a high slew rate to provide an adequate bandwidth for transmission of the signals.